Post Date: 10 January 2020

The carbonaceous aerosol levels still remain a challenge in the Beijing-Tianjin-Hebei region of China: Insights from continuous high temporal resolution measurements in multiple cities

Abstract

A one-year hourly measurement (from December 1, 2016 to November 30, 2017) of organic carbon (OC) and elemental carbon (EC) in fine particulate matter was performed using semi-continuous OC/EC analyzers in Beijing, Tianjin, Shijiazhuang and Tangshan in the Beijing-Tianjin-Hebei (BTH) region in northern China.

The annual average concentrations (± associated standard deviation) of OC and EC were 11.0±10.7 and 3.4±3.3, 12.0±9.8 and 3.1±3.6, 22.8±30.6 and 5.4±6.5, and 12.1±9.6 and 3.5±3.6 μg/m3 for Beijing, Tianjin, Shijiazhuang and Tangshan, respectively. The carbonaceous aerosol levels (the sum of the OC multiplied by 1.6 and the EC concentration) accounted for 36 %, 33 %, 41 % and 31 % of the PM2.5 mass values in the four sites, indicating that the carbonaceous fraction provides a substantial contribution to the fine particle mass.

There was a strong seasonality in the OC and EC concentrations with higher levels in autumn and winter and lower ones in spring and summer.

A pronounced double-peak diurnal pattern occurred for OC in all four seasons at all four cities, with one peak occurring between approximately 6:00 and 10:00, which could have resulted from the combination of vehicular emissions in the morning and secondary OC (SOC) formation caused by incremental solar radiation, and the other peak occurring during the evening traffic rush hour, which even lasted until the next early morning. Also EC and the OC/PM2.5 and EC/PM2.5 ratios exhibited the pronounced double-peak diurnal pattern, indicating that there was a substantial impact from traffic emissions.

Our OC and EC levels were lower than published data from the past two decades for the BTH region, implying some effect of recent measures for improving the air quality.

The regression lines of OC versus EC were examined for each of the four seasons and over the full year for the four sites. Significant correlations (p<0.001) were found throughout the study period with high slopes and correlation coefficients in winter, but low slopes and correlation coefficients in summer.

The estimated SOC based on the minimum R squared (MRS) method accounted for 14.7-47 %, 21-74 %, 21-55 % and 17.0-74 % of the OC for Beijing, Tianjin, Shijiazhuang and Tangshan, respectively, with annual percentage SOC/OC ratios of 29 %, 47 %, 38 % and 48 %. There was a tendency for our ratios to be larger than previous ratios obtained for the BTH region in the past decade, which is mainly attributable to the difference in estimating SOC.

There were obvious differences in the potential source regions of OC and EC among the four cities. Obvious prominent potential source areas of OC and EC were observed for Beijing, which were mainly located in the central and western areas of Inner Mongolia and even extended to the Mongolian regions, which is different from the findings in previous studies. For all sites, adjacent areas of the main provinces in northern China were also found to be important potential source areas.